The present invention generally relates to a printing blanket construction, and more particularly, to an improved end design for a printing blanket adapted to be used with a magnetic printing cylinder. The type of printing blanket to which is referred herein is used primarily in offset lithographic printing, but may also find utility in other fields of printing.
In the art of offset lithography, a rotary cylinder is covered with a printing plate which normally has a positive image area receptive to oil-based inks and repellant to water and a background area where the opposite is true. The printing plate is rotated so that its surface contacts a second cylinder covered with an ink-receptive printing blanket having an elastomeric surface. The ink present on the image surface of the printing plate transfers, or offsets, to the surface of the printing blanket. A continuous paper web or sheet stock to be printed is then passed between the blanket-covered cylinder and a rigid back-up cylinder to transfer the image from the surface of the printing blanket to the paper.
The blanket-covered cylinders have to accommodate a mechanism to hold the printing blanket on the cylinder surface. This mechanism is typically located in a gap extending axially or longitudinally along the cylinder. Typically, these gaps will have a circumferential distance (width) of at least 0.18 inches. As a result, for every revolution of the blanket-covered cylinder, that portion of the web which passes the printing cylinder gap is not imprinted. This results in an increase in operating costs which may be attributed to several factors including the need for printing cylinders having larger diameters to achieve the desired page length and the wasted paper produced from such gaps. Moreover, the cylinders in a typical offset printing apparatus rotate at high speeds and with substantial pressure between the cylinders, thereby providing a structural environment in which the gaps cause shock and vibrations degrading the printing quality and contributing to printing apparatus wear. In addition, the gaps destroy the symmetry of the cylinders which is an undesirable condition for high speed rotation. As a result, the rotational speed of the blanket-covered cylinders must be limited. Accordingly, it would be desirable to have printing apparatus which provide a narrow gap on the blanket-covered cylinder.
In response to such problems, cylinders to which a printing blanket is held magnetically have been proposed. Magnetic cylinders in combination with magnetic printing blankets have been used in attempts to produce a narrow gap functionally approaching a continuous printing blanket. For example, Clifton, U.S. Pat. No. 3,616,145 provides a magnetized printing blanket adapted to be self-attached to a printing cylinder and Pickard et al, U.S. Pat. No. 3,730,092 provide a magnetic blanket retained on a printing cylinder using a plate and a top sheet, both of which are mounted in the gap of the printing cylinder. While offset printing apparatus having the combination of a magnetic printing blanket and a magnetic printing cylinder may result in a slightly narrower gap, they do not exhibit sufficient holding capabilities for reliable operation in high speed offset printing operations nor do they achieve a narrow gap functionally approaching a continuous printing blanket. More specifically, the centrifugal forces acting on a printing blanket during high speed printing can exceed the magnetic holding power between blanket and cylinder causing a unidirectional shift in the location of the gap with respect to the cylinder. As a consequence, printing quality is degraded, and operating costs are increased.
Thus, it would be desirable to have offset printing apparatus which would, in addition to providing a more narrow gap approaching that of a continuous blanket, maintain the gap of a blanket-covered cylinder in a constant position. Accordingly, there remains a need in the art for a printing blanket having an improved design which will result in a narrow gap on a printing cylinder and yet which maintains the blanket and the gap in a constant position.